In an electrophotographic image forming apparatus such as a laser printer, an electrostatic copying machine, a plain paper facsimile machine or a printer-copier-facsimile multifunction machine, an image is generally formed on a surface of a sheet such as a paper sheet or a plastic film through the following process steps.
First, a surface of a photoreceptor body having photoelectric conductivity is evenly electrically charged and, in this state, exposed to light, whereby an electrostatic latent image corresponding to an image to be formed on the sheet is formed on the surface of the photoreceptor body (charging step and exposing step).
Then, toner (minute color particles) preliminarily electrically charged at a predetermined potential is brought into contact with the surface of the photoreceptor body. Thus, the toner selectively adheres to the surface of the photoreceptor body according to the potential pattern of the electrostatic latent image, whereby the electrostatic latent image is developed into a toner image (developing step).
Subsequently, the toner image is transferred onto the surface of the sheet (transfer step), and fixed to the surface of the sheet (fixing step). Thus, the image is formed on the surface of the sheet.
Further, a part of the toner remaining on the surface of the photoreceptor body after the transfer of the toner image is removed, for example by a cleaning blade (cleaning step). Thus, the photoreceptor body is ready for the next image formation.
In the charging step out of the aforementioned process steps, a charging roller is used, which is kept in contact with the surface of the photoreceptor body to evenly electrically charge the surface of the photoreceptor body.
Widely used as the charging roller is a semiconductive roller having an outer peripheral surface to be kept in contact with the surface of the photoreceptor body, at least the outer peripheral surface being made of a crosslinking product of a semiconductive rubber composition.
The semiconductive rubber composition for the semiconductive roller is generally prepared by blending a rubber component, including at least an ion conductive rubber such as an epichlorohydrin rubber, a crosslinking component for crosslinking the rubber component, and the like.
Minute particles such as of silica or titanium oxide are externally added to the toner for controlling the fluidity, the electrical chargeability and other physical properties of the toner.
However, the external additive particles and toner particle pieces resulting from pulverization of the toner particles during repeated image formation (hereinafter referred to generally as “external additive particles”) cannot be completely removed from the surface of the photoreceptor body by the cleaning blade or the like and, therefore, the external additive particles left unremoved are gradually deposited on the outer peripheral surface of the charging roller constantly kept in contact with the surface of the photoreceptor body during the repeated image formation.
The deposited external additive particles are liable to influence the electrical chargeability and other physical properties of the photoreceptor body and adhere to a formed image, resulting in imaging failure.
Particularly, where the charging roller is produced by forming the semiconductive rubber composition into a tubular body, crosslinking the tubular body and dry-polishing an outer peripheral surface of the tubular body to a predetermined surface roughness, streaky polishing marks are formed axially on the outer peripheral, surface and, during the repeated image formation, the external additive particles are deposited linearly along the polishing marks. This results in imaging failure such as transverse streaking in a formed image (Patent Document 1 and the like).
It is noted that the invention disclosed in Patent Document 1 mainly relates to a developing roller, and minute polishing marks formed on the developing roller hardly influence the image formation and, therefore, cause no problem.